The present invention generally relates to a scanning apparatus using a rotary polygon mirror, and in particular to a scanning apparatus using a rotary polygon mirror in use for a laser printing machine, a laser beam scanner or a copying machine.
It is well known that a scanning apparatus using a rotary polygon mirror has widely been applied to many kinds of optical apparatuses. A laser printing machine is one example of such optical apparatuses. The laser printing machine can print information at an extremely high-speed. A scanning apparatus using a rotary polygon mirror used in the laser printing machine deflects a laser beam emitted by an optical source and scans a photosensitive drum to form electrostatic latent images thereon. A conventional scanning apparatus mainly includes a rotary polygon mirror driven by a motor, a laser diode unit consisting of a laser diode and a collimator lens, a cylindrical lens, a transparent plate, and an f.theta. lens. The laser diode unit and the cylindrical lens are arranged in an incident optical path of the scanning apparatus, whereas the f.theta. lens is arranged in a reflection optical path. The transparent plate is arranged in both the incident and reflection optical paths. A laser beam emitted by the laser diode unit passes through the cylindrical lens and the transparent plate, and is deflected by the rotary polygon mirror. A deflected laser beam passes through the transparent plate and the f.theta. lens, and then scans the photosensitive drum, so that electrostatic latent image are formed on the drum.
During the scanning operation, the rotary polygon mirror is rotated by the motor at a high-speed in the range from approximately a few thousand to twenty thousand rotations per minute. The high-speed rotating motion of the rotary polygon mirror causes noise due to the rotary polygon mirror cutting air in its vicinity as well as high frequency sounds generated at a bearing for coupling the rotary polygon mirror with the axis of the motor. Therefore, the rotary polygon mirror of the conventional scanning apparatus is enclosed by a cover specifically designed for the rotary polygon mirror, as will be described later. The rotary polygon mirror enclosed by the cover is mounted on a housing which accommodates an optical lens system composed of the laser diode unit, the cylindrical lens and the f.theta. lens.
However, the conventional scanning apparatus using the rotary polygon mirror has the following disadvantages. First, the total cost of the scanning apparatus becomes expensive, because there must be manufactured specifically the cover designed for enclosing the rotary polygon mirror. This also results in an increase in the total number of parts. Further, an additional space for accommodating the cover in the housing is required. Moreover, the operation for positioning the rotary polygon mirror enclosed by the cover with respect to the optical lens system is very complicated.
Secondly, flare light due to an undesirable reflection of the laser beam by the transparent plate forms noises on the images on the drum.